A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 236 means of a pair of intermediate gates, into two sections or lengths, so that it can be accommodated to the reception of large or small vessels, as the case may be, with the minimum expenditure of water during the process. The quantity of water withdrawn from the dock will be a matter for consideration if the operation of locking be very prolonged. The following table, modified from one in Rankine’s work on Civil Engineering, shows the results of lockage under varions conditions. Let L denote a lockful of water—that is, the volume contained in the lock chamber, between the upper and lower water levels; let B denote the volume displaced by a boat. TABLE XXIII.—Lockage. Lock Found Water Discharged. Lock Left One boat undocking, ,, docking,. Two n boats docking and) undocking alternately, / Series ofwboats undocking, JJ »> »» ,, ,, docking, . Two series, each of | boats, the first undock-. ing, the second docking, J Enipty. Full. Empty or full. /Undocking, full.) (Docking, empty./ Empty. Full. Empty or full. Full. L-B. L+B. n L. nL-n B. (n - 1) L - « B. «L + » B. (211-1) L. Empty. Empty. Full. /Undocking, empty. (Docking, full. Empty. Empty. Full. Full. Against the advantages afforded by the use of a lock have to be set the greatly increased cost of construction over that of a simple entrance and the additional space required. The projection of the inner end of a lock into the dock itself is a plan which, though often adopted, is attended by a decrease in the utilisable lengtli of quay and in the convenience of berthing. In large ports, the combination of a simple entrance with one or more locks is no uncommon feature. The former is used for docking large ships during the period in which there is free communication between the dock and the river ; the latter, which are often in two widths, are brought into active service when the entrance is closed, or they may be utilised contem- poraneously as subsidiary entrances. At Barry there is a single entrance, 80 feet in width, and a lock adjoining, 647 feet by 60 feet. The recently constructed entrances, at the north end of the Liverpool dock system, comprise an entrance,* 100 feet wide, an 80-foot lock, 130 feet long, and a 40-foot lock, 165 feet long. These are all parallel in direction, pointing up-stream, but at Kidderpur docks, advantage has been taken of a bend in the waterway to arrange a lock, 400 feet by 60 feet, in an up-stream * This entrance is, strictly speaking, a loek, being provided with two pairs of ebb- gates ; but it is rarely, if ever, used as sueh, the chamber being only 130 feet long, and the provision of two pairs of gates is really a safeguard against the contingencies previously referred to.